During a storm along the Danish coastline some 5,000 years ago, two fishermen waded into the cold waters of the Baltic Sea to shore up a system of fishing weirs against the poor weather. Or at least that’s what archaeologists think happened, after finding two sets of human footprints preserved in a dried-up fjord on the island of Lolland in Denmark. Preliminary dating of the sediment and minerals in the sand where the prints were found suggests they were made back in the Stone Age, when a system of fishing weirs helped feed a nearby community.

For more than a year, a team of archaeologists from the Museum Lolland-Falster has been working at a fevered pace to recover artifacts and fossil evidence from the planned site of the Fehmarn Belt Fixed Link. Construction is slated to begin next year on the underwater tunnel, which will eventually connect the Danish island of Lolland with the German island of Fehmarn across the Baltic Sea and enable travel between Denmark and Germany in a mere 10 minutes. Now, while excavating the sands of a dried-up fjord on Lolland, the scientists have uncovered an unexpected find: two sets of ancient human footprints and a tool known as a fishing fence, which dates back to around 3,000 B.C.

Constructed of a thin switch of hazel suspended between two larger sticks, such fences were lined up next to each other to form a long, continuous weir system, in which fish would get stuck. Though archaeologists have found similar tools before, this was the first time human footprints had been uncovered with them. Terje Stafseth, an archaeologist with the museum, said in a press release that: “This is really quite extraordinary, finding footprints from humans. Normally, what we find is their rubbish in the form of tools and pottery, but here, we suddenly have a completely different type of traces from the past, footprints left by a human being.”

The depth and position of the footprints, as well as their close proximity to the three-foot-long system of fishing weirs, led the scientists to conclude that a storm had caused the weirs to flood. Experts from the museum believe the two sets of prints, one larger than the other, were made by fishermen attempting to safeguard the weirs during the storm. As the fishermen’s feet sunk deeper into the seabed, sand stirred up by the storm surge covered them, preserving their prints over time.

Lars Ewald Jensen, the Museum Lolland-Falster’s project manager for the Fehrman Link project, told Live Science that the footprints he and his team found showed fine layers of mud and sand positioned one top of one another in a clear pattern. Preliminary dating of the sediment and minerals in the sand suggest the prints go back to the Stone Age, somewhere between 5,000 B.C. and 2,000 B.C. The museum’s experts are now making imprints, or flat molds, of the prints to preserve them for posterity. As excavations of the tunnel continue, they hope further tests on the prints will yield more information.

At the time the prints were made, the level of the Baltic Sea was rising thanks to melting glaciers in northern Europe, and local Stone Age settlements used the fjords (sea inlets typically formed by glacial erosion) for water-related activities such as fishing, as well as offering sacrifices to the sea. The museum team also uncovered several skulls belonging to domestic and wild animals on the beach near the fjord, which they believe were part of offerings made by local farmers who lived in the area from around 4,000 years ago.

In 1872, the Baltic Sea flooded during a storm, killing 80 people on the island of Lolland alone. Shortly after the flood, a dyke was built along Lolland’s southern coast to prevent future storm surges, leaving the region’s fjords dry. The above-ground facilities of the planned Fehmarn Belt Fixed Link will cover up the dried fjord beds, including the one where the prints were found.